Method and apparatus for identifying heartbeat

ABSTRACT

The invention relates to a method and an apparatus for identifying a heartbeat. In the method, an arterial pressure signal is measured in the form of multichannel measurement, and the required signal-processing operations, such as filtration, are carried out if necessary. The method also comprises carrying out signal detection and decision-making concerning the acceptance as a heartbeat signal. According to the invention, the signal detection includes multichannel channel-specific detection for the purpose of identifying signal components of different channels, and the obtained channel-specific detected channel signals are used as input data during the decision-making stage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for identifying a heartbeat, themethod comprising measuring an arterial pressure signal in the form of amultichannel measurement, and carrying out signal-processing operations,such as filtration, if necessary, and performing signal detection anddecision-making concerning the identification of a signal as a heartbeatsignal.

The invention also relates to an apparatus for identifying a heartbeat,the apparatus comprising a multichannel sensor, preferably asignal-processing means for processing a signal obtained from themultichannel sensor, detection means for detecting a signal, anddecision-making means for identifying the signal as a heartbeat signal.

2. Brief Description of the Prior Art

Prior art conventional arrangements for heartbeat measurement performedon an arterial pressure pulse are based on the use of one sensingelement, in other words the measurement is carried out via one channel.The problem is, however, that heartbeat measurement, based on the use ofone sensing element, from an arterial pressure pulse is only accuratewhen the subject is at rest and interference is minimal. The movement ofa limb, a tendon or a muscle creates significant interference in thesignal measured by the sensor, which may result in an unsuccessfulmeasurement. The problem is aggravated by the similarity in frequencybands and waveforms of the interference and the actual signal in anindividual measurement channel, which makes it more difficult todistinguish the signal from interference.

Arrangements utilizing multichannel measurement are also known.Multichannel measurement of a pressure pulse provides better capacitythan a one-channel method. In a multichannel pressure pulse signal, theinterference is mainly common-mode or similar amongst differentchannels, which enables elimination of interference caused by movementof the subject. A known method is disclosed in U.S. Pat. No. 5,243,992.The method disclosed in this reference utilizes a multichannel tonometersensor. The signal processing utilizes known methods for decreasinginterference signals, such as band-pass filtration and subtraction of anaverage of signals of all channels from each channel signal. In U.S. PatNo. 5,243,992, the heartbeat detection is based on the selection of thebest signal channel. The pulse in the selected channel is accepted as aheartbeat pulse by means of a correlation function. A few precedingpulse amplitudes are used as the selection criteria for the channel.Such selection of one channel leads to uncertain operation duringinterference situations and when the sensor moves with respect to theartery. Due to the high interference level, the method based on theselection of one channel may lead to the selection of a channel otherthan the one with the best heartbeat signal. Also, when the heartbeat ofa moving person is measured, the movement of the sensor at the wristwith respect to the artery may transfer the heartbeat signal to adifferent channel, and therefore the signal may be lost for a whileuntil a channel providing a good signal is located again. Also, the useof mathematical operations, such as a correlation function, requires agreat deal of calculation capacity, in which case implementation of asignal processor is required, which in turn leads to a large size andgreat consumption of energy. Other known multichannel measurementmethods are disclosed in EP 404,594 and SE 425,290.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a new type of methodand system avoiding the problems of the prior arrangements.

In order to achieve this, the method according to the invention ischaracterized in that in the method the signal detection includesmultichannel channel-specific detection for the purpose of identifyingsignal components of different channels. The channel-specific detectedchannel signals obtained are used as input data in the decision-makingstage.

In order to achieve this, the apparatus according to the invention ischaracterized in that the detection means are arranged to carry outmultichannel channel-specific detection on several different channels.The decision-making means are connected to perform identification of aheartbeat signal from among several channel signals detectedspecifically for each channel.

The invention provides several advantages. The arrangement according tothe invention provides better reliability and capacity with the use ofmore than one signal channel for the detection of a heartbeat or pulse.Movement of the sensor with respect to the artery does not necessarilyremove the signal from all the channels, as might happen in aone-channel method. The algorithm according to the method of theinvention can be implemented in a simple manner with, for example, ageneral-purpose processor or a digital or preferably analog ASIC, whichprovides a wrist-watch-like small and light apparatus operated with abattery. The preferred embodiments of the invention and other moredetailed embodiments accentuate the advantages of the invention.

The invention will be described in greater detail below by means ofexamples with reference to the accompanying drawings, in which

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an apparatus according to the invention,

FIG. 2 shows one signal-processing channel of FIG. 1,

FIG. 3 shows a multichannel pressure pulse signal,

FIG. 4 shows a multichannel pressure pulse signal after the wrist hasbeen rotated,

FIG. 5 shows a heartbeat pulse component of a pressure signal, anddetection thresholds used for detecting it,

FIG. 6 shows a pulse rate meter wrist strap placed on the wrist andcomprising an apparatus according to the invention integrated therein.

DETAILED DESCRIPTION OF THE INVENTION

A block diagram of the apparatus/method according to the invention isshown in FIG. 1. The invention relates to an apparatus for identifying aheartbeat. The apparatus comprises a multichannel sensor 2 andpreferably signal-processing means 3, which most preferably includesfiltration means 3, for processing a signal obtained from themultichannel sensor 2. In FIG. 2, the filtration means 3 is divided intotwo blocks 3 a and 3 b.

The apparatus for identifying a heartbeat also comprises detection means4 for carrying out signal detection, and decision-making means 5 foridentifying a signal as a heartbeat signal.

FIGS. 1 and 6 show that the multichannel sensor 2 comprises, forexample, eight sensing elements 21 to 28. Correspondingly, FIGS. 3 and 4illustrating pressure pulse signals S1 to S8 show channels C1 to C8corresponding to the sensing elements 21 to 28. The multichannel sensor2 may be any sensor, for example a PVDF sensor based on thepiezoelectric effect or, for example, an electromechanical film (EMF)sensor based on an electret film. FIG. 6 shows a pulse rate meter wriststrap 30 wrapped around a person's wrist 29 with an associated apparatus1 according to the invention for identifying a heartbeat. The block 30that represents a pulse rate meter or some other so-called hostapparatus is also shown in FIG. 1. The pulse rate meter 30 or some otherhost apparatus includes calculation means 31, a display 32 and possiblyalso other components typically found in wrist-worn pulse rate meters,such as a Telemetric heartbeat receiver, a keypad for inputting data,and connection means for transferring data to a personal computer. Theconnection between the apparatus 1 and the apparatus 30 may be wired orwireless. If the connection is wired, the apparatus 1 and the apparatus30 are most preferably integrated into the same apparatus. Referencenumeral 33 denotes a wired connection.

In the situation shown in FIG. 6, the connection is wireless, i.e.between the apparatus 1 and the apparatus 30 there is a telemetricremote readable connection 34 via remote readable means 35 and 36, suchas remote readable coils 35 and 36. The remote reading technique. whichis based on magnetic inductive coupling, is based on oscillatingcircuits producing magnetic oscillation.

The apparatus 1 is characterized in that the detection means 4 arearranged to carry out multichannel channel-specific detection forseveral different channels C1 to C8 and that the decision-making means 5are connected to identify a heartbeat signal from among the numerouschannel signals specifically detected from each channel. In the method,a pressure signal in an artery 40 is measured in the form of amultichannel measurement, and preferably the signal-processing, such asfiltration, is carried out, as well as signal detection anddecision-making concerning the identification of a signal as theheartbeat signal. Signal detection in the present invention includesmultichannel channel-specific detection for the purpose of identifyingsignal components of different channels. The channel-specific detectedchannel signals obtained are used as input data in the decision-makingstage in the block 5. In the preferred embodiment of the invention, themultichannel channel-specific detection includes identification of theheartbeat pulse component of the pressure signal as part of theidentification of the signal components of the different channels.

FIG. 6 shows that the multichannel sensor 2 is positioned such that atleast one sensing element of the sensor 2, for example elements 23 to26, is placed over the artery 40, and that at least one other sensingelement of the sensor 2, for example elements 21 to 22 and 27 to 28, isnot situated over the artery 40. Thus, in addition to a measurementcarried out from a point with a heartbeat signal, such as by elements 23to 26, the method also comprises measurement from a point with noheartbeat signal, such as by elements 21 to 22 and 27 to 28.Correspondingly, in addition to the formation of at least one channelsignal having a heartbeat pulse in the channel-specific detection, atleast one channel signal with no heartbeat pulse is also formed. Themeasurement of a channel containing no heartbeat, e.g. channel C8, isutilized in the signal filtration, since by means of the signal S8 ofthe channel C8 having no heartbeat it is possible to determine theaverage value of the common-mode interference signal.

As shown in FIG. 6, signals obtained from the multichannel pressuresensor are thus filtered both specifically for each channel, such as byindividual band-pass filtration in block 3 a, and with respect to theother channels, such as by canceling the common-mode interference signalin block 3 b. Stated differently, the procedure is such that the signalof each channel C1 to C8 is strengthened and the interference is reducedby subjecting the signal S1 to S8 of each channel C1 to C8 to individualband-pass filtration in step 3 a, and by subtracting the value thatrepresents the common-mode interference signal of the signals onadjacent channels in step 3 b. The order in which these steps 3 a, 3 bare carried out may be different from that disclosed above.

In a preferred embodiment of the invention, a pressure pulse is detectedfrom the signal of each channel by detecting positive peaks H andnegative peaks L of the filtered signal within a time window W, which ispreferably predetermined. In the preferred embodiment, the detection ofthe channel signal utilizes a fixed or changing time window W. Detectionof a peak value is simple and advantageous. In order to simplify thearrangement, it is noted with reference to FIGS. 2 and 5 that thedetection utilizes detection thresholds THR1 and THR2, and that thesedetection thresholds are either fixed and common to all channels,adaptive and common to all channels or adaptive specifically for eachchannel. FIG. 5 may represent for example signal S3 as regards itsheartbeat pulse component SC3. Detection threshold THR1 shows thedetection threshold of the positive peak value H, and detectionthreshold THR2 shows the detection threshold of the negative peak valueL. With reference to FIG. 2, it is stated that this time delay betweenthe peak and the minimum, i.e. the time window W, is most preferablyabout 100 to 200 milliseconds.

To further improve the efficiency and adaptivity of the invention, inthe preferred embodiment, the apparatus 1 comprises weighting means 6for weighting the contribution of one or several channels differentlyfrom the other channels. The apparatus 1 most preferably comprises arule block 7 that controls the weighting means 6. The rule block 7 isalso connected to and controls the decision-making block 5.

In the preferred embodiment, the channel signals C1 to C8 are weightedfor the purpose of decision-making depending on how the heartbeat pulse,e.g. SC3, has occurred previously in the aforementioned channels, e.g.in channel C3. The weighting can be carried out either before thedetection or most preferably after the detection, as shown in thefigures.

In the preferred embodiment of the invention, the weighting utilizesweighting coefficients p1 to p8 that can be updated, as shown in FIG. 1.The weighting coefficients are then updated on the basis of empiricalrules stored in the block 7.

In the preferred embodiment of the invention, an effective empiricalrule determines that if a signal having similar characteristics has beendetected in adjacent channels in previous measurements, during thedecision-making these channels are weighted heavily more than the otherchannels. FIG. 3 shows that channels C3 to C6 comprise similar signalsin adjacent channels, and therefore the weighting coefficients p3 to p6are updated so that in later measurements, either during the samemeasurement period or in a separate subsequent measurement, the channelsC3 to C6 will be weighted more heavily.

In the preferred embodiment of the invention, if a signal having similarcharacteristics has been detected in one or more non-adjacent channelsin previous measurements, during the decision-making these channels areweighted less heavily than the other channels. In such a case, theweighting coefficients are updated so that in subsequent measurements,either during the same measurement period, or in a separate measurementthese non-adjacent channels will be weighted less heavily.

The weighting is therefore most preferably adaptive and emphasizes thefavors those channels from which a pressure pulse signal has beenprevious detected. The decision-making concerning the identification asa heartbeat signal is based on the number of channels C1 to C8 in whichthe desired signal has occurred on average. The weighting coefficientsp1 to p8 are continuously updated on the basis of the empirical rules,so that as the sensors 21 to 28 move with respect to the artery 40, thealgorithm is capable of rapidly locating the channels where the desiredsignal occurs. For instance, when the wrist is turned, the sensors movewith respect to the artery and the heartbeat signal moves from onechannel to an adjacent channel. Thus, the heartbeat signal does not jumpfrom one channel to another randomly, but moves in a relativelypredictable manner as shown in the transition from FIG. 3 to FIG. 4.Specifically, the movement of the sensing elements 21 to 28 of thesensor 2 with respect to the artery 40 and the resulting transfer of theheartbeat signals to different channels is illustrated by a comparisonof FIGS. 3 and 4. In FIG. 3, the heartbeat signals can be detected onchannels C3 to C6. In FIG. 4, as a result of a rotation of the wrist 29or some other movement or some other interfering factor, the heartbeatsignals have moved to other channels and they now constitute signals S5to S8 on channels C5 to C8.

In the preferred embodiment of the invention, the decision concerningthe identification of a heartbeat signal is carried out in thedecision-making stage on the basis of the cross-correlation of signalsin two or more adjacent channels. In a preferred embodiment, thedecision is carried out in the decision-making stage on the basis ofcommon-mode signals occurring simultaneously in two or more adjacentchannels. According to the Applicant, the comparison of two or moreadjacent channels is an effective manner to reach a decision. Forinstance, FIG. 3 shows that adjacent channels C3 to C6 containcommon-mode signals S3 to S6 that occur simultaneously and indicate aheartbeat.

When a heartbeat signal cannot be identified on the basis of monitoringtwo or more adjacent channels, in the next step of the invention, thedecision in the decision-making stage is carried out on the basis of asignal that occurs regularly in one channel, such as when only onechannel detects a heartbeat and the remaining channels are eithernon-responsive (i.e., flat-line) or detect pulses that are well below apredetermined noise floor or interference threshold.

The apparatus may be implemented by means of analogue or digitaltechnology, microprocessor technology, or a combination thereof. Asregards the components, they may consist of discrete components,integrated circuits or a combination thereof.

Even though the invention is described above with reference to examplesaccording to the accompanying drawings, it is clear that the inventionis not restricted thereto, but it can be modified in several ways withinthe scope of the inventive idea disclosed in the appended claims.

What is claimed is:
 1. A method for identifying a heartbeat comprisingthe steps of: measuring an arterial pressure signal in the form ofmultichannel measurement; and performing signal detection anddecision-making concerning the identification of a signal as a heartbeatsignal; wherein said signal detection includes multichannelchannel-specific detection for the purpose of identifying signalcomponents of different channels, and said obtained channel-specificdetected channel signals are used as input data in said decision-making,and wherein said heartbeat signal is detected from a signal of eachchannel by detecting positive and negative peaks of said signal of eachchannel within a time window.
 2. A method according to claim 1, furtherincluding the step of weighting said channel signals for the purpose ofsaid decision-making based on the heartbeat signal previously occurringin said channels.
 3. A method according to claim 2 wherein saidweighting utilizes weighting coefficients that can be updated.
 4. Amethod according to claim 3 wherein said weighting coefficients areupdated on the basis of empirical rules.
 5. A method according to claim4 wherein adjacent channels having similar signals are weighted morethan other channels during said decision-making.
 6. A method accordingto claim 4 wherein non-adjacent channels having similar signals areweighted less than other channels during said decision making.
 7. Amethod according to claim 2 wherein said weighting is carried out aftersaid channel-specific detection and before said decision-making.
 8. Amethod according to claim 1 further including the step of performingsignal detection and decision-making concerning the identification of asignal as a non-heartbeat signal.
 9. A method according to claim 1further including the step of forming at least one channel signal withno heartbeat signal.
 10. A method according to claim 1 wherein said timewindow is a fixed or changing time window.
 11. A method according toclaim 1 wherein said detection utilizes detection thresholds, saiddetection thresholds being selected from the group consisting of fixedand common to different channels, adaptive and common to differentchannels and adaptive specifically for each channel.
 12. A methodaccording to claim 1 further including the step of filtering saidsignals both specifically for each channel and with respect to the otherchannels.
 13. A method according to claim 1 further including the stepof reducing the interference of each channel by subjecting the signalsof all of said channels to band-pass filtration and subtracting acommon-mode interference signal from said signal of each channel.
 14. Amethod according to claim 1 wherein said signals are obtained from theartery at the wrist by using a wrist strap containing a sensor.
 15. Amethod for identifying a heartbeat comprising the steps of: measuring anarterial pressure signal in the form of multichannel measurement; andperforming signal detection and decision-making concerning theidentification of a signal as a heartbeat signal; wherein said signaldetection includes multichannel channel-specific detection for thepurpose of identifying signal components of different channels, saidobtained channel-specific detected channel signals are used as inputdata in said decision-making, and a decision is made in saiddecision-making step on the basis of cross-correlation of signalsoccurring in two or more adjacent channels.
 16. A method for identifyinga heartbeat comprising the steps of: measuring an arterial pressuresignal in the form of multichannel measurement; and performing signaldetection and decision-making concerning the identification of a signalas a heartbeat signal; wherein said signal detection includesmultichannel channel-specific detection for the purpose of identifyingsignal components of different channels, said obtained channel-specificdetected channel signals are used as input data in said decision-making,and a decision is made in said decision-making step on the basis ofcommon-mode signals occurring simultaneously in two or more adjacentchannels.
 17. A method for identifying a heartbeat comprising the stepsof: measuring an arterial pressure signal in the form of multichannelmeasurement; and performing signal detection and decision-makingconcerning the identification of a signal as a heartbeat signal; whereinsaid signal detection includes multichannel channel-specific detectionfor the purpose of identifying signal components of different channels,said obtained channel-specific detected channel signals are used asinput data in said decision-making, and a decision is made in saiddecision-making step on the basis of a signal occurring regularly in onechannel when said heartbeat signal cannot be identified on the basis ofmonitoring two or more adjacent channels.
 18. A method for identifying aheartbeat comprising the steps of: measuring an arterial pressure signalin the form of multichannel measurement; and performing signal detectionand decision-making concerning the identification of a signal as aheartbeat signal; wherein said signal detection includes multichannelchannel-specific detection for the purpose of identifying signalcomponents of different channels, said multichannel channel-specificdetection including identifying a heartbeat pulse component of saidarterial pressure signal as part of said signal components of saiddifferent channels, and said obtained channel-specific detected channelsignals are used as input data in said decision-making.